1. Field of the Invention
This invention relates to the analysis of optical fiber drawdown zones and more particularly to techniques for monitoring the drawdown zone during drawing processes.
2. Description of the Prior Art
The successful application of optical fibers to long distance transmission systems requires strict adherence to specific design parameters. For example, such fibers must be fabricated with very close tolerances on the fiber diameter, the index of refraction distribution within the fiber, and the number of imperfections, such as microcracks, along the fiber. Because of the strict tolerances placed on the various fiber specifications, detailed techniques for measuring these parameters have been developed.
In U.S. Pat. No. 3,879,128 issued to H. M. Presby on Apr. 22, 1975, a technique is described for monitoring the diameter of an optical fiber. This technique involves scattering light off the optical fiber, detecting such scattered light, and analyzing the resultant interference pattern to extract from it the magnitude of the fiber diameter. Such a process takes on added significance when one realizes how critically the fiber diameter affects the transmission properties of the fiber. In certain transmission modes the fiber diameter must be constant along the length of the fiber to within less than 1 percent, while in other applications the fiber diameter must vary periodically along the fiber length with well defined amplitude and periodicity. Such diameter specifications may be monitored in detail using the Presby technique. However, it should be realized that when the fiber is monitored during the drawing process, the degree to which the information obtained can be effectively used in a feedback loop to control the drawing parameters is limited. The reason for this is that the drawing is done at very rapid speeds and many meters of imperfect fiber will be fabricated before a correction is fully realized. Other schemes for detecting fiber parameters suffer from similar limitations. Namely, they detect the parameters in the fiber after it is drawn, and consequently have limited application to configurations which vary or alter the drawing parameters in response to a feedback signal.
Geometrical optics, upon which the Presby technique is predicated, is a field of physics which was very well understood even before the dawn of the twentieth century. Among the many phenomena that appear in the optics literature is a scattering process which results in areas of increased light intensity known as caustic rays. Such caustic rays have been put to only limited use in the analysis of optical surfaces. An example of such an application may be found in the International Journal of Solid Structures, Volume 12, pages 377-389, where caustic ray analysis is described as a technique for measuring the optical quality of glass surfaces. That technique, and techniques similar to it, have however met with only limited success.